Radiotherapeutic Fundamentals of Image-Guided HDR Brachytherapy

Abstract

High-dose-rate (HDR) brachytherapy after image-guided catheter implantation is an established treatment option for oligotopic metastases and for tumors of the liver, lung, or kidney; it is considered superior to thermoablation. We compared image-guided HDR brachytherapy (iBT) with modern external radiotherapy techniques, such as Cyberknife®, volume-modulated arc therapy (VMAT), and TomoTherapy®, which we used for radiosurgery. To that end, we estimated tumor-control probabilities (TCP) and normal-tissue complication probabilities (NTCP) and applied this theoretical framework for radio-ablation, applying a single dose of 20 Gy. A large therapeutic ratio requires an increase in TCP in conjunction with a low NTCP by minimizing volumes of the surrounding organs at risk in the high-dose (5–10 Gy) and low-dose (1–2 Gy) ranges. In particular, large volumes with medium and low doses might be harmful in organs, such as liver, lung, and kidney, which are radiobiologically described as parallel organs. We calculated optimum dose distributions for intrahepatic lesions of 1–5 cm diameter for iBT, Cyberknife, VMAT, and Tomotherapy and examined the advantages and disadvantages of each technique. For small lesions, i.e., up to 3 cm diameter, we found a clear advantage for iBT; this advantage decreases for diameters ≥4 cm. Image-guided HDR brachytherapy is by far the most effective treatment method, even for extended lesions, and yields the highest therapeutic ratios. Cyberknife comes second. However, for lesions ≥4 cm VMAT and Tomotherapy turn out to be serious competitors because of their excellent feasibility and tolerability, and their short treatment times.